The invention is particularly applicable to windshield washer systems for automobiles but is not limited to that art area. In Bauer application, U.S. Ser. No. 618,252, filed Oct. 16, 1975 now U.S. Pat. No. 4,157,161 issued June 5, 1979 and assigned to the assignee hereof, various forms of liquid fan spray generation systems are disclosed, including, one as disclosed in U.S. Pat. No. 4,052,002, assigned to the assignee hereof, of which the inventor hereof is a co-inventor, and incorporated herein by reference. In such fluidic oscillation systems, the distance between the power nozzle (e.g., the entrance aperture for the power jet) and the outlet or throat area of the system (e.g., the exit aperture) and the ratio of the cross-sectional area of the power nozzle to the outlet throat area control the fan angle. The distance between the power nozzle and the throat is a function of the fan angle, the fan angle being inversely related to that distance. Thus, if the fan angle is selected, then the area ratio of the throat outlet to the power nozzle is established. For example, the cross-sectional area of the output throat should be approximately two and one-half times the area of the power nozzle. However, in the oscillator as shown in U.S. Pat. No. 4,052,002 as well as conventional fluidic oscillators, the liquid jet from the power nozzle must fill up the cavity in order to set up alternate vortices in the interaction region. Initially, a coherent fluid jet travels from the power nozzle through the throat in a straight stream. The power jet must therefore expand sufficiently to fill the throat before the interaction region and the oscillation feedback channels begin to fill. Vortices are formed on either side of the jet of the fluidic oscillator but two vortices cannot exist simultaneously with equal intensity. Thus, as one vortex becomes dominant, the power stream will be diverted against the opposite wall and the oscillation begins.
The power jet velocity at which the throat is filled is the threshold velocity level and is directly proportional to threshold pressure level. When the weather turns cold, the surface tension and viscosity of the fluid increase so the cold fluid power jet does not expand readily so as to fill the cross-sectional area of the throat or outlet. In such case, no fan spray develops and the power jet stream passes through the outlet throat to impinge directly on the windshield. An increase in the velocity (an increase in the threshold pressure) by increasing pump pressure will provide the additional expansion and may be implemented to start the oscillation, but it may not be successful in producing, for example a full fan angle. However, these same factors, i.e., increased surface tension and viscosity influence the fluid velocity as the pump will deliver a lower pressure fluid for the same energy input level.
Without the invention hereof, the same fluid (water plus an antifreeze, in a ratio of 50:50; CH.sub.3 O.sub.8 --H.sub.2 O at 3 CP (centipoise)) at 0.degree. F., the threshold pressure for oscillation is greater than 30 pounds per square inch. With the invention hereof, the same fluid at 0.degree. requires a threshold pressure of about equal to or somewhat greater than three pounds per square inch in a fluidic oscillator of the same character and size. Thus, according to the present invention, the same oscillator geometry or silhouette has increased or improved cold start oscillation capabilities while at the same time maintaining the low pressure start capabilities, and avoids the use of a large pump.
According to the invention, the cross-sectional area of the outlet throat is maintained substantially constant but the height to width ratio of the throat (the aspect ratio (H/W)) is increased (by increasing H and diminishing W) while maintaining the cross-sectional area substantially constant. Whereas the cross-section of the throat outlet approaches a square while maintaining substantially the same area, the cross-section of the power nozzle remains essentially the same. The fan pattern angle is proportional to the ratio of the throat area to the power nozzle and by increasing the aspect ratio, there is a reduction in the outlet throat width so that the jet does not need to expand as much to fill the throat. All of this leads to a greatly improved cold weather starting capability for the oscillator. In the absence of the present invention, cold weather oscillation is substantially reduced and/or eliminated so that only a single concentrated jet stream impinges upon the windshield of an automobile, for example.